This invention relates generally to a rotatable control valve assembly for an auxiliary hydraulic power steering system in an automotive vehicle.
Rotatable controls valve for standard hydraulic power steering systems develop so called hissing noises at the valve control edges. The loudness of such noises depend on the ratio of pressures upstream and downstream of the control edges. Also, such noises become louder as the fluid pressure head increases and the noise is transmitted through the conventional metallic drive connection between the vehicle steering wheel and the steering control valve.
A rotary steering control valve having means for eliminating hissing noises developed at the valve control edges, is already known as disclosed in U.S. Pat. No. 4,335,749 to Walter owned in common with the present application. Such control valve is so constructed that inlet control edges located directly downstream of inlet bores raise pump pressure continuously to the requisite operating pressure level. Such inlet control edges are provided with bevel surfaces or champfers for control of pressure. With respect to pressure control by the rotatable core valve member involved, pressurize fluid or oil is regulated within three control grooves of a coacting valve sleeve, such grooves being respectively relieved through three of six inlet control edges, by way of example. The other three inlet control edges establish an unrestricted flow path to another groove in the valve sleeve in fluid communication with an active servomotor pressure chamber. The volume of fluid displaced from the servomotor is always conducted through the the aforementioned control grooves, pressure relieved through six constant flow throttling passages formed as bores in fluid communication with the oil reservoir tank through additional valve sleeve grooves for return flow. The throttling passage bores are responsible for pressure build-up of fluid under control of the three control edges to regulate diversion of a portion of the pump inflow stream to the accumulated return flow paths, thereby significantly decreasing any reduction in pressure at such inlet control edges. Disturbing noises otherwise developed from the inflow of pressurized fluid is thus avoided. However, the accumulation of return flow fluid in the flow modulating positions of rotary core valve member, increases energy imbalance accommodating an increase of pressure build-up in the active pressure chamber of the servomotor to overcome the return flow accumulation. Further, the formation of parallel throttle passages in the core valve member and the additional grooves for return flow, are responsible for relatively high production costs.
It is therefore an important object of the invention to provide a rotatable valve assembly for a hydraulic steering control system of aforementioned type which operates without hissing noises, and yet avoids accumulation of return flow fluid from the servomotor chamber opposing the activated pressure chamber thereof during steering control movement.
Yet another object of the invention, in accordance with the foregoing object, is to provide a constructionally simpler control valve assembly.